1. Field Of The Invention
This invention relates to rapid curing polyurethane elastomers. More particularly, this invention provides polyurethane elastomers which are curable and releasable at a relatively low temperature and in a short period of time.
2. Description Of The Prior Art
The so-called casting polyurethane elastomers, which have been widely used in rollers, belts, solid tires, and the like, because of their excellent mechanical characteristics, wear resistance, resistance to oils, and the like, includes two types. One type is the TDI (toluene diisocyanate) prepolymer type which employs MOCA (3,3'-dichloro-4,4'-diaminodiphenylmethane) as a curing agent and employs polytetramethylene ether glycol (hereinafter simply referred to as PTMEG), polyester diol, polyoxypropylene glycol (hereinafter simply referred to as PPG) and the like as the polyol. The other type uses the so-called semi- one shot process in which 1,5-naphthylenediisocyanate or the diphenylmethanediisocyanate/polyester diol system is employed and glycol is employed as the curing agent.
However, the two types mentioned above have drawbacks such as curing thereof takes several hours at about 100.degree. C. and workability is extremely low.
In both types mentioned above, the pot life on mixing the prepolymer with the curing agent can be shortened by use of a catalyst, but it is quite impossible to shorten the curing time to several minutes. Moreover, it is necessary to raise the temperature of the raw materials up to around 100.degree. C. because of the high viscosity of the prepolymer, high melting point of MOCA, and the like. However, no high pressure casting machine, to which the raw material at such a high temperature is applicable, is available at present. Even if such a high pressure casting machine should be available, it is impossible to shorten the curing time to several minutes.
With respect to high density polyurethane foam, which is also called micro-cellular elastomer, and is used as individual automobile parts such as bumpers, and the like, Japanese published examined patent application nos. 22711/1972, 38587/1971 and 1386/1972 each disclose a process in which curing is effected more rapidly at a lower temperature compared with the so-called casting elastomer mentioned above. All the processes disclosed as above follow the so-called one shot process, in which modified diphenylmethanediisocyanate (MDI) is used as the polyisocyanate.
With respect to rapid curing polyurethane elastomers, Japanese Japanese published unexamined patent application nos. 118795/1974 and 125199/1976 disclose the use of a prepolymer as the polyisocyanate. The latter patent application discloses the use of PTMEG in the prepolymer and as a curing agent.
In accordance with the processes disclosed as above, however, it is very difficult to simultaneously satisfy both rapid curability and the physical properties required of the cured product. No elastomer with high physical properties equal to an elastomer obtained from PTMEG/TDI prepolymer represented by Adiprene L-100 (produced by E. I. Du Pont de Nemours & Co.), where MOCA is employed as a curing agent, has been obtained.
The present inventors made an extensive study to obtain an elastomer having rapid curability as well as physical properties equal to those of a casting type elastomer represented by Adiprene L-100 produced by E. I. Du Pont de Nemours & Co. which is cured on heating by use of MOCA. The present inventors discovered that such an elastomer can be obtained by satisfactorily selecting the composition of a prepolymer so that the viscosity thereof may be lowered to such an extent as to be applicable to a casting machine at a temperature of from room temperature to the relatively low temperature of about 60.degree. C.